The corrosion of steel pipes, tanks and other equipment under thermal insulation has been a significant problem in the petrochemical, refinery and other industries. A highly effective corrosion control method is required due the presence of corrosive electrolytes, high temperature resistance and the need for long term reliability since corrosion under insulation (CUI) cannot be visually inspected without removing the insulation material. The significantly high costs of corrosion inspection and repairs may result in a huge financial burden to the owners due to the accessibility, the requirement of the thermal jacket removal and re-installation, and manufacturing downtime. When the corrosion problems are ignored, serious accidents may occur, and the frequency of such accidents has been increasing recently worldwide.
The causes of CUI are typically caused by the intrusion of rain water, deluge system water, wash water, or condensation. The insulation system should be water tight; however, the failure of water tightness occurs in some areas, resulting in water intrusion. The facilities are typically located near oceans, so that the rain water is contaminated with chlorides. The petrochemical factories also release sulfates into the air, so that rain water may also be contaminated with sulfates.
When the corrosive contaminated water penetrates under the insulation and contacts high-temperature steel pipes, tanks and equipment, the water evaporates and leaves corrosive contaminants. Further water intrusion increases the concentration of the substances under the thermal insulator. In addition, condensation on cold pipelines, or cyclic hot-and-cold pipelines, is another source of the CUI electrolyte. The intrusion of contaminated air under the insulator also occurs regardless of the intactness of the thermal insulator jackets. As a result, the corrosivity of the condensed water under the insulation increases with time.
Some steel components under thermal insulation may be protected by temperature resistant di-electric coatings. Various types of applied di-electric liquid coatings have been developed to handle high temperatures to some extent. To apply these liquid coatings, the steel surface preparation is the most critical part of the process. Blasting is required to remove contaminants, such as chlorides and other salts, and to provide the proper anchor pattern for the coating. However, because most thermally insulated components are located in hazardous areas, such surface preparation must be conducted with safety and environmental constraints. The surface preparation using blasting method is not feasible in highly congested pipeline areas because there is not enough room for the blasting work in confined spaces. If the coating is damaged for any reason, it cannot protect the exposed steel, resulting in corrosion pits.
Recently, thermally sprayed aluminum coatings using 1100 aluminum alloy were used to protect steel components from CUI. The passive film on the aluminum coating protects the substrate steel as a barrier coating. Aluminum is known to provide galvanic cathodic protection anode when sufficient chlorides exist in the electrolyte (e.g. seawater) because they break the passive film on the aluminum surface. When the aluminum passive film is disrupted, the potential of the aluminum shifts in more negative direction and can protect the steel by galvanic cathodic protection. When the electrolyte for CUI does not contain sufficient chlorides, the aluminum sprayed coating cannot protect the substrate steel as a galvanic anode at low temperatures. In addition, when the sprayed aluminum coating is exposed to highly chloride contaminated CUI electrolyte, the coating turns to sacrificial anode. The coating is then consumed from the exposed surface by self-corrosion, resulting in shorter life.
In addition, similar to liquid coating, the steel surface preparation using blasting is essential for bonding thermal sprayed aluminum coating. The application of thermal sprayed aluminum requires a skilled technician because the speed of spray gun movement, maintaining proper spraying distance and angles, aluminum wire feeding speed, etc., are important considerations. When the structures are located in fire hazardous area, the working area must be shut down during the application. In some congested areas, it is not even possible to apply the coating on site. As a result, the cost of the application is extremely high.
Zinc-rich primers and di-electric coatings may be used together. When the steel surface is exposed, the zinc-rich primer provides galvanic protection. However, the typical life of zinc-rich primer is short in highly corrosive electrolytes because zinc exhibits a low electrical capacity. In addition, the zinc particles in the primer are rapidly consumed in high-temperature electrolytes. When a sufficient amount of zinc particles in the primer are consumed, they can no longer protect a steel substrate. As such, the corrosion protection afforded by zinc rich primer is limited due to the short life.
Commonly, steel hot water tanks are galvanically protected using magnesium anodes, because it was thought any other sacrificial anodes, such as zinc and aluminum, could not provide protection in chloride-free water. However, the efficiency of a magnesium anode is typically less than 50 percent, and even less in chlorinated water due to the self-corrosion.